Reply #11: Well that is the magic question.

[Statistical]

Solar insolation is about 1000w per m^2.
So 20% efficient panel would be 200w per m^2.
13KW / 200w = 65 m^2 = 700 square feet.

As far as how cheap? Well that is the magic question.

The panels make up about 60% of total system cost so that is where any future cost reductions will come from.

Nanosolar says they can get costs down to $1 per watt.

Currently high quality panels are $3 or more a watt.
Inverters run about another $0.50 - $0.70 per wattt.
Mounting hardware & wiring another $0.60 - $0.80 or so a watt.
Labor really depends on how much you want to pay people.

Higher efficiency panels have lower labor costs.

Just so you know 13KW DC is not needed for the average home, 5 to 6KW is more than enough for most areas.